Many prior art systems exist for detecting the presence of particles or size of particles in a fluid, such as a supply of potable water. For example, U.S. Pat. No. 5,438,408 entitled Measuring Device and Method for the Determination of Particle Size Distributions by Scattered Light Measurements discloses the use of a charge coupled device (CCD) camera. U.S. Pat. No. 6,061,130 entitled Apparatus for Determining the Particle Size Distribution of a Mixture discloses an apparatus that includes a CCD matrix. By identifying particles by predetermined parameters, such as diameter or cross-sectional area, such systems can ascertain the presence or absence of unwanted harmful bacteria in a water sample which are known to be within a predetermined range of diameters.
Some of these systems have also been known to be useful in analyzing other fluids such as blood and blood products. Typically, identifying particle populations in accordance with some parameter, for instance particle size or particle cross-section, allows a parameter distribution to be ascertained. In a water supply the goal may be to determine the number of particles of various sizes that are present in a representative sample.
Detection systems most often employ the use of computers or powerful processor-based systems coupled to one or more CCD or pixel arrays of detecting elements, which detect the presence of one or more particles projected upon a portion of the array of charge coupled elements. Often thousands of frames of information are collected. Within a single frame more than a single particle may be detected; therefore, the software is programmed to find clusters of pixels, indicating the presence of a particle, and to determine a number of pixels, or a pixel total, for the cluster. Some software can determine instances where portions of particles overlap and determine the size of each particle.
In each successive frame, images of the particles contained within an optical sampling volume are projected onto the pixel array by a magnification system. These images of the particles are randomly distributed on the array depending on the positions of the particles in the sampling volume. In order to produce the parameter distribution information, the parameter value corresponding to each pixel total must be determined. When the number of pixels is large, a simple scaling factor, which depends only on the pixel size and the magnification, gives accurate results. However when the number of pixels is small, that is, when only very few detectors sense the presence of at least a portion of a particle, this scaling factor becomes increasingly uncertain and depends more strongly on the image location and on a detector sensitivity threshold.
Image location error results from the fact that the pixel total measured for a particular value of a particle image depends on the location of the image with respect to the pixel grid. This can be understood more clearly by way of example considering a particle which image has a cross-sectional area corresponding to exactly one array element. Such an image can overlap, either completely or partially, from one to four array elements, and therefore can potentially trigger from one to four array elements, or pixels, depending on the image position with respect to the detector array and the sensitivity threshold setting of the detector elements.
Whether a partially exposed element of the detector array will trigger a pixel count depends on the detector sensitivity threshold, which contributes to uncertainty of the relation between a particle size and a pixel total count of its image. This uncertainty, which decreases with increasing the pixel total count, is hereafter referred to in this specification as a pixelation error. A minimum number of 9 pixels are normally considered to be required to achieve approximately 30% accuracy in a single image measurement. In applications, and depending on a data processing technique used, this minimum pixel count threshold Nmin of reliable image detection can be either larger or less than 9.
A method of at least partially obviating negative effects of the pixelation errors on obtaining a parameter distribution from digital images of a sample of particles was disclosed in a U.S. patent application Ser. No. 10/653,133 filed Sep. 3, 2003 by a same inventor, which is incorporated herein by reference. The method involves post-processing of the measured statistical data containing pixel count per image using pre-determined probability coefficients relating a pixel total count to a particle size, which can be obtained for example by measuring a statistically large sample of particles of same diameter and analyzing statistical distribution of the pixel count per image. Using this method, reliable statistical information about particle size distribution in a sample containing statistically significant number of small particles can be obtained using a small number of pixels per image without significantly suffering from the effects of the pixelation error normally associated with using a small number of pixels.
An ability of the prior art systems to measure large samples of small particles is however limited by the used magnification system; regardless of a particular value of the minimum pixel count per image adopted for the system, as the particles of interest become smaller, a proportionally larger magnification factor is normally required to reliably detect smaller particles, leading to an undesirable reduction of a sample size that can be analyzed in one measurement.
For in-line operation or, in applications where a large number of samples must be analyzed, it is desirable that measurements be made in the shortest possible time. For example it would be desirable to analyze a sample in several minutes and not in several hours. Furthermore, it is desirable that a single measurement at a single magnification provides information, i.e. the number of particles in each of a specified range of equivalent diameters for the particles having the largest possible range of sizes.
To ensure that that the pixelation error is small, a sufficiently high magnification may be selected so that the images of the smallest particles occupy a sufficient number of pixels, no less than a fixed minimum pixel total Nmin. In magnification systems used for imaging in conventional microscopy, the optics is designed to provide a magnified image, which has minimum distortion and closely resembles the particle under examination. In such systems, the optical sampling volume over which non-distorted images may be obtained is a product of a field of view and a depth of focus of the optics used; at sufficient magnification it is very small, and become smaller as the magnification is increased. Typically, a system with 15 times magnification used to image 2 micron particles would have a depth of focus of approximately 2.5 microns and a field of view on the pixel array of approximately 0.5×0.5 mm. The resultant small optical sampling volume severely limits the number of particles in a flowing stream, which may be in-focus for measurement at a given time. By way of example: the time required to analyze a typical sample of 1 cc, using a magnification such that a 2.5 micron particle occupies two hundred 7.5×7.5 micron pixels, is approximately 5 hours. Furthermore as magnification is increased, the size of the largest particle, which may be imaged without incurring a significant probability that its image will overlap with the edge of the pixel array, is reduced; for the magnification value used in the example, this upper limit is approximately 50 microns. As a result, conventional microscopy has not been commercially used heretofore to make measurements on particle populations in flowing streams.
For maximum measurement speed and maximum parameter measurement range it is desirable that largest sample volumes could be measured in a single measurement using a smallest possible magnification factor that provides sufficient number of pixels in the image of the smallest particle to be included in the characterization of the population.
It is an object of this invention, to provide a relatively fast and inexpensive imaging system whereby a small magnification factor can be used to image a small particle in a large sample volume.